Wireless systems are becoming a fundamental mode of telecommunication in modern society. In order for wireless systems to continue to penetrate into the telecommunications market, the cost of providing the service must continue to decrease and the convenience of using the service should continue to increase. In response to increasing market demand, radio standards around the world have been proliferated based upon digital modulation schemes. Consequently, it is often advantageous to have a receiver that is capable of communication using more than one of these standardized techniques. In order to do so, it is necessary to have a receiver that is capable of receiving signals that have been modulated according to several different modulation techniques.
Existing receivers are implemented using double conversion (or heterodyne) receiver architectures. A double conversion receiver architecture is characterized in that a received radio-frequency (RF) signal is converted to an intermediate frequency (IF) signal, which is subsequently converted to baseband. In addition, gain control is also typically applied at the IF. However, double conversion receivers have the disadvantage of utilizing a great number of analog circuit components, thus, increasing the cost, size, and power consumption of the receiver.
A direct conversion receiver, also sometimes called a zero-IF receiver, provides an alternative to the traditional double down conversion architecture. This is particularly attractive for the use in wireless systems, especially in handsets, since direct conversion receivers lend themselves more easily to monolithic integration than heterodyne architectures. Also, direct conversion exhibits immunity to the problem of image since there is no IF.
However, there are design issues associated with the direct conversion architecture. The most serious problem is a direct current (DC) offset in the baseband, which appears in the middle of the down-converted signal spectrum, and may be larger than the signal itself. This phenomenon is caused by local oscillator leakage and self-mixing. Furthermore, I/Q mismatch, occurring in the quadrature down-conversion, can lead to corrupted signal constellation, thereby increasing the number of bits in error, due to the differences which may occur in the I and Q signal amplitudes.